A firewall protects open ports on your machine. A browser does not have any open ports.

Of course they have vulnerabilities, but a firewall won’t protect you from them.

For ssh, sure.

But a browser? No way.

It’s not.

If you have a house, it has a door which you can use to access everything inside.

If you have a linux install with no services running on it, it has no doors, and thus doesn’t need any door locks. And if it does have services running on there (which run publicly), it now has doors, sure, but getting one of those doors open doesn’t guarantee access to the whole house - usually it’s gonna be an empty room